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Hawaiian Islands Synoptic Discussion and Guidance

673
FXHW60 PHFO 190627
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
827 PM HST Tue Feb 18 2020

.SYNOPSIS...
Gusty easterly trade winds will continue through Thursday, then
gradually trend down and shift out of the southeast over the
upcoming weekend as a front approaches the region. Although most of
the showers will focus over windward and mountain locations, some
will make it into leeward areas occasionally with the strong trades
in place. Light and variable winds will become a possibility by
Sunday as the tail-end of the front moves into the area and
diminishes.

&&

.DISCUSSION...
The latest surface analysis showed 1031 mb high pressure centered
north-northwest of the state, placing a strong pressure gradient
over the islands. Local winds are responding and have steadily
increased through the afternoon and early evening hours today.
Observations around the state showed peak gusts ranging from 35 to
50 mph. Strongest winds will affect exposed areas such as
mountain ridges and points between the islands, as well as leeward
areas downslope of the mountains. In addition to the winds, a
band of shallow moisture is supporting showery conditions this
evening over Maui County and Oahu. These showers have been light
with very little accumulations, mostly focusing over windward and
mountain locations (occasionally making it into leeward areas
briefly).

Guidance suggests high pressure will build to around 1035 mb by
Wednesday night, as it shifts to an area due north of the state. A
combination of very dry mid-level air, a strong subsidence
inversion positioned around 6000 to 7500 ft and a strengthening
pressure gradient will support the windy conditions holding into
Thursday. Periods with enhanced shower activity associated with
shallow bands of moisture within the trades moving through and
weak pulses aloft dropping southward through the islands are
expected through Friday. Although most of this activity will
focus over windward and mountain locations, some will spill over
into leeward sections with the strong winds in place.
Accumulations will remain light.

The extended guidance depicts a large-scale pattern shift across
the central Pacific Friday through the weekend as a broad upper
trough and cold front approach from the northwest. Low-level flow
will gradually veer out of the southeast and weaken, potentially
enough for light and variable winds by the end of the weekend into
Monday. As the winds veer, showers will transition to the
southeast slopes of Maui and the Big Island and interior areas
across the western end of the state through the afternoon hours as
sea breezes setup.

The front will dissipate Monday with high pressure building to
the north in its wake. A return of a more typical trade wind
pattern will be possible beginning around Tuesday. A lingering
band of moisture associated with the diminished front, however,
may support decent windward shower coverage continuing over
portions of the state through midweek.

&&

.AVIATION...
High pressure north of the state will keep strong and gusty trade
winds blowing through Wednesday. Clouds and showers will favor
windward and mauka areas, although showers will also reach leeward
locations more regularly, particularly during the night and early
morning hours. Showers will bring MVFR cigs/vsbys to windward
areas at times, while VFR conditions prevail in leeward locales.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Kauai, Oahu, Molokai and Maui. These
conditions will likely continue through the night.

AIRMET Tango is in effect for low level turbulence over and
downwind of the terrain, as well as strong surface winds across
the entire state. These conditions are expected to continue
through Wednesday at least.

&&

.MARINE...
A strong high about 750 nm northwest of Kauai will move east at 20
kt over the next few days. The pressure gradient south of the high
will produce strong to near-gale force easterly trade winds over
Hawaiian waters with winds reaching gale force in areas where the
wind accelerates around the islands. A Gale Warning is in effect
through Thursday evening for the windy zones around the Big Island
and Maui and a Small Craft Advisory (SCA) is in effect for all
other Hawaiian Coastal Waters.

Later in the week as the high moves farther away to the east the
trade winds will weaken. A front will approach from the northwest
starting Saturday. Winds will shift out of the southeast, then
south ahead of the front. The front is expected to move into the
far northwest offshore waters Sunday night with strong northeast
trade winds returning behind the front.

The strengthening trade winds will cause surf to exceed the High
Surf Advisory threshold of 8 feet along east facing shores
Wednesday. This surf will subside late in the week as the trade
winds weaken.

The current northwest swell is subsiding. A new small northwest
swell will peak Wednesday night or Thursday, then gradually lower
through Friday. A slightly larger northwest swell is expected to
arrive late Friday and Friday night, peak Saturday, then lower
slowly Sunday and Monday. A series of small south swells will
continue through the coming weekend.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 6 AM Wednesday to 6 AM HST Thursday for Kauai-
Oahu.

Wind Advisory until 6 PM HST Thursday for Molokai-Lanai-
Kahoolawe-Maui Windward West-Maui Leeward West-Maui Central
Valley-Windward Haleakala-Leeward Haleakala-South Big Island-Big
Island North and East-Kohala.

Small Craft Advisory until 8 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Gale Warning until 6 PM HST Thursday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

&&

$$

DISCUSSION...Gibbs
AVIATION...Jelsema
MARINE...Donaldson

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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